The problems that may arise when carbon fibers are accidently released into the environment have been well publicized. Because of its electrical conductivity, carbon, and particularly carbon fiber when released as the result of a fire, might come into contact with electrical and electronic systems and cause unforeseen malfunctions. Because of their very light weight, graphite fibers can float in the air like dust particles and, if they come to rest on electrical circuits, can cause power failures, blackouts, shorts or arcing that can damage equipment. The high electrical conductivity of the carbon fibers has been identified as the prime factor in their effects on electrical equipment, with other properties such as small fiber diameter, generally short length, and low density being important contributory factors.
This disclosure is directed to the application of the techniques of electropolymerization and electrodeposition developed for interphase modification of carbon fiber composites toward a solution to the problems of airborne carbon fiber fragments.
This disclosure results from an investigation of electrochemical coating of graphite fibers by high temperature resistant polymers, organophosphorus and other flame retardant polymers, and organometallic or inorganic materials which function as precursor coatings capable of forming or being converted to highchar, relatively nonconductive residues on graphite fibers during burning of graphite fiber composites. It seeks a solution to the problems arising from the accidental release of electrically conductive graphite fibers from graphite-polymer composites exposed to fire and combustion. It involves the coating of graphite fibers by electropolymerization and electrodeposition, the preparation of composites from the thus coated fibers, and an evaluation of the effectiveness of the precursor coatings in enhancing char formation and fiber clumping during combustion of the composite. It adds a new dimension to interphase modification in composites, studies of which have mainly focused on strength properties. Suitable techniques have previously been developed and reported by the inventors for electropolymerization of monomers and electrodeposition of polymers on graphite fibers, showing that significant improvements in composite strength and toughness result when the coated fibers are incorporated in an epoxy matrix. In the present disclosure, the novel technique of interphase modification by electrocoating processes is applied to the formation of suitable coatings on graphite fibers which result not only in improved composite properties but also in reduced electrical hazards in the event of fire.
The general and specific advantages of the electrochemical processes for coating graphite fibers are:
(a) Techniques are available and previously have been developed for electrocoating commercial graphite fibers used as reinforcement;
(b) Interphase modification of carbon fibers by electrocoating processes has been shown to be effective in improving composite mechanical properties;
(c) Electropolymerization of monomers and electrodeposition of polymers can be conducted on a variety of graphite fiber electrodes;
(d) Chemical and physical structure and other properties of the polymer coatings can be varied by employing different monomers--vinyl, acetylenic, cyclic or other types, and different ionizable polymers;
(e) Further variations in chemical structure and properties can be achieved by copolymerization;
(f) Chemical bond formation, i.e., grafting of polymer to the fiber, may be introduced;
(g) Cross-linked polymer coatings of increased modulus can be formed by employing appropriate multifunctional monomers;
(h) Additional parameters can be controlled, e.g., by varying the nature of solvents and electrolytes employed;
(i) The thickness of the polymer coating can be controlled by modifications of process parameters;
(j) Intermediates for the formation of high temperature resistant polymer coatings such as acetylene terminated polyimides, and polyamic acids are capable of electropolymerization or electrodeposition;
(k) Flame retardant vinyl monomers such as chloro and bromostyrene, 2, 3-dibromopropyl acrylate and 2, 3-dibromopropyl methacrylate are available for electropolymerization to produce precursor coatings.
(1) Vinyl monomers containing flame retardant phosphonate groups, phosphonium salts such as tetrakis (hydroxymethyl) phosphonium sulfate (THP sulfate) are available which can be used in coating graphite fibers by electropolymerization or electrodeposition.
The present invention has been directed to the application of coating materials on carbon fibers by use of electrochemical coating techniques and to a study of the behavior of the carbon fiber polymer composites which include the coated carbon fibers under combustion conditions. The thermal oxidative behavior of the composites was investigated by thermogravimetric analysis. The ability of different types of coating materials or coating material precursors to reduce the potential for accidental release of carbon fibers has been compared.
Organophosphorus as well as inorganic phosphorus-containing flame retardant compounds have been found by us to inhibit combustion by converting organic compounds into char during burning. This is accomplished by formation of phosphoric acid, which promotes char formation. The phosphoric acid which is formed from the phosphorus-containing compounds also forms an insulating layer shielding the unburned organic matter. The resulting phosphorous-containing coatings on the graphite fibers enhance the formation of clumps of fibers during combustion of graphite-polymer composites.
Acetylene terminated polyimide precursors are readily available and have been found by us to polymerize electrochemically on carbon fibers. Since polyimides are more temperature resistant than most other polymer resins (unlike epoxies), polyimide coatings on carbon fibers not only provide protection to a higher temperature, but also remain on the fiber fragments in the event of release. The residual polyimide coating on the fiber fragment, being nonconductive, serves the same purpose as residual char.
The presence of long chain or multiple organic groups in the selected coating compounds is favorable for compatibility or coreaction of the precursor coating with the matrix polymer. For example, the hydroxyl groups in THP have been found by us to react with the epoxy groups of an epoxy resin.
Titanate coupling agents are available which also contain (ionizable) phosphate or pyrophosphate groups, and polymerizable vinyl or acrylic functions in addition to other aliphatic and aromatic groups, amino groups, etc. These organotitanates were found to possess the attributes required to form, by our electrochemical techniques, desirable precursor coatings on graphite fibers which lead to char formation, relatively non-conductive residues and fiber clumps upon exposure to fire. They also provide for effective graphite fiber-polymer interaction, although by a slightly different mechanism than in the case of the mineral fiber composites for which they have been developed. The chemical link between the titanium and the graphite fiber surface can be attributed to the probability of occurrence of transesterification with --C--OH functions on the graphite fiber surface. The attraction of the oppositely charged organotitanium species to the graphite surface during electrodeposition provides a compacted layer of organotitanate on the fiber, as in the case of electrodeposition of polymers. This improves fiber-matrix adhesion. The organic/functional groups of the organatitanate coating further promotes efficient interaction and bonding with the matrix.
The over-all objective of the present invention is to propose coating materials or coating material precursors for graphite fibers which can be applied by electrochemical techniques, which can maintain or preferably, improve composite properties, which would convert to a high electrical resistance coating in situ, and which also result in fiber "clumping" during fire and explosion, thereby to provide a solution to the problems arising from the electrical effects of release of conductive fiber fragments into the environment. Specifically, it is the objective of this invention to:
(1) form high temperature resistant polymer coatings such as polyimides, polyquinoxalines, etc., by electropolymerization of appropriate intermediates carrying acetylene terminal groups;
(2) form polyimide coatings by electropolymerization or electrodeposition of aminophthalic acid, polyamic acids and other suitable intermediates;
(3) form a grafted or coating layer of organophosphorous compounds or polymers by electropolymerization and electrodeposition of suitable organophosphorus monomers and other phosphorous containing compounds onto graphite fibers;
(4) form polymer coatings by the electropolymerization of organotitanates carrying polymerizable groups such as vinyl, or acrylic functional groups;
(5) form organophosphorus-titanate layers by electrodeposition of organotitanates carrying ionizable phosphate or pyrophosphate groups;
(6) form coatings of boric acid and borates by electrodeposition on graphite fibers;
(7) extend the possibility of forming similar precursor coatings by appropriate selection of other classes of monomers and intermediates for electropolymerization and electrodeposition.